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Original description, (original code is PS 1):

Currently, errors from the platform (e.g. device disconnected), are sent
directly to the renderer (AudioOutputStreamClient). When moving to the audio
service, this becomes suboptimal. It is tricky to signal an error occurring
before the ClientPtr is set up, and sometimes we may not want it torn down due
to a platform error (in case we want to try some sort of recovery operation
such as restarting the audio service). This CL instead signals the error to the
browser (AudioOutputStreamObserver), which signals it to the client through the
new AudioOutputStreamProviderClient interface. That interface also gets the
"stream created" callback as a method rather than a mojo response, so that we
will be able to call it multiple times in the future.

Diff PS 1 -> PS 2 removes a call to a sequence checker that was accidentally
added when rebasing the original CL.

Diff PS 2 -> last PS fixes an issue when pepper would call Start before the
stream is created. This is solved by storing the state that the client
expects in the MojoAudioOutputIPC object. That object then initializes the
state when the stream has been created. As an aside, it is a great preparation
for restarting stream, since MojoAudioOutputIPC will be able to restore the
volume and playing state without involvement from AudioOutputDevice.

Bug: 787806,830493,834242
Change-Id: Iee3a3902e9086cc9df4c5d210e43e65c162273fe
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Reviewed-on: https://chromium-review.googlesource.com/1023397
Commit-Queue: Max Morin <maxmorin@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553965}